1. Field of the Invention
The present invention relates to a projection aligner and an exposure method using the same. More particularly, the present invention relates to a projection aligner for projecting a mask pattern and an exposure method using the same.
2. Description of the Background Art
A conventional projection aligner will be first described in the following.
FIG. 4 is a schematic view showing the structure of a conventional projection aligner. Referring to FIG. 4, the conventional projection aligner demagnifies a pattern on a reticle (mask) 11 and projects it onto a photo resist (photosensitive material) on a wafer 20, and has a light source 13, an illumination optical system 12 from light source 13 to reticle 11, and a projection optical system 110 from reticle 11 to wafer 20.
Specifically, projection optical system 110 has a lens barrel 101 and a plurality of lenses 102a to 102e arranged in lens barrel 101.
In the exposing operation, a light beam emitted from light source 13 is first projected through illumination optical system 12 onto reticle 11 on which a pattern is formed. The pattern image thus produced is projected onto the photo resist applied to wafer 20 through lenses 102a to 102e in projection optical system 110.
Here, the reference character AX in the figure denotes the optical axis of the light beam emitted from light source 13.
In manufacturing semiconductor integrated circuits, projection aligners are extensively employed to form fine patterns. For the recent demand for finer patterns, the exposure wavelength has become smaller to the g ray (436 nm), the i ray (365 nm) and further to the KrF excimer laser light beam (248 nm). Further, in order to form patterns of at most the wavelength of an exposure light beam, such super-resolution technique as the use of a phase shift mask is required.
Since the phase shift mask is a technique to improve resolution by interference of a light beam in an opposite phase, the light beam interference has to be large to some extent. Thus, the interference between mask apertures is generally made larger to attain its effect in the super-resolution technique. It is therefore necessary to carry out exposure by increasing the interference of illumination (reducing the ".sigma. (coherency) value"). In this case, an image may be formed by the interference using only part of a pupil, and this increases the influence of aberration as distortion of a lens and deteriorates the imaging property.
Although the lens aberration is attributable to the accuracy of polishing the lens, lens form distortion due to the lens weight after assembling a projection aligner is also one of the major causes in the conventional projection aligner.
For example, for a parallel plate lens 2 arranged in lens barrel 101 as shown in FIG. 5, its own weight causes the central portion of the lens to bend more than the peripheral portion in a direction in which gravity works. When lens 2 does not bend, the path A of a beam passing through lens 2 is linear. However, the path B of a beam passing through bent lens 2 is refracted by lens 2 and is not linear. Thus, the beam is displaced, causing aberration.
Such aberration causes the following effects in forming device patterns.
Pattern displacement due to comma is one example. Hole patterns and interconnection patterns are used in LSIs (Large Scale Integrated circuits). As shown in FIG. 6, the structure in which a hole 26 for electrically connecting a lower layer interconnection 22 and an upper layer interconnection 28 passes through an intermediate layer interconnection 24 is generally used. In this structure, hole 26 is formed in interlayer insulating films 23, 25. Hole 26 is filled with a conductive layer 27, and lower layer interconnection 22 is formed on a silicon substrate 21, for example.
In the structure shown in FIG. 6, when relative displacement between hole 26 and intermediate layer interconnection 24 is caused by aberration while a pattern is transferred, conductive layer 27 filling hole 26 and intermediate later interconnection 24 may be electrically short-circuited.
When the displacement due to aberration during pattern transfer is uniform in the exposure field, the influence of displacement due to aberration is substantially eliminated if exposure is carried out by deliberately shifting the amount of displacement during overlay exposure. In an actual stepper, however, the amount of displacement (including the directions) due to aberration is non-uniformly distributed in the field. Further, since displacement varies from one pattern to another (due to the pitch of a line/space pattern, for example), eliminating this effect is considered to be difficult. Even in a portion having the same patterns and the same image fields, the hole patterns and the interconnection patterns are relatively displaced by changing the exposure method (for example, modified illumination and normal illumination).
In order to form patterns not to cause such an electric short circuit, therefore, the space between hole 26 and intermediate layer interconnection 24 has to be widely designed. Then, the patterns can not be arranged closely, the chip size becomes larger, the number of production per wafer decreases, and production is lowered.
The typical amount of relative displacement for an actual stepper is considered to be 20-30 nm with a device having a 0.20 .mu.m rule. Since the amount of displacement (conventional "overlay error" not caused by aberration) when the same patterns are overlayed and exposed under the same exposure conditions is 40-50 nm, such displacement between patterns that can not be ignored is caused by aberration.
FIG. 3 of C. M. Lim "Analysis of nonlinear overlay errors by aperture mixing related with pattern asymmetry", SPIE Vol. 3051, pp. 106-115 shows the distribution, in an exposure field, of displacement between large size (.about.10 .mu.m .quadrature.) patterns due to a difference in the type of illumination (modified illumination). FIG. 3 (a) is the measured amount of displacement, and FIG. 3 (b) shows the amount of displacement without a parallel movement component and a magnification error component (the amount of displacement which can not be corrected by a stepper when the second layer is exposed). It can also be seen from this figure that the amount of displacement which can not be corrected is at most 30 nm.